Conventionally, transfer molding is performed for encapsulating electronic devices wherein molding compound is introduced as a solid pellet into a pot of a molding system and melted with the application of heat and pressure to a liquid state. The liquefied molding compound is then forced by a plunger into runners connected between the plunger and molding cavities to enter into molding cavities. An example of a transfer molding system is disclosed in U.S. Pat. No. 5,520,874 entitled “Compressible Mold Plunger”.
The disadvantage of conventional transfer molding systems is that the use of epoxy molding compound is inefficient, resulting in wastage. Such wastage is typically found in the culls, runners and gating system of the mold, which are discarded after molding. The ratio of the molding compound wasted as compared to the molding compound used may be as high as 0.2:1.
In modern packaging technology, semiconductor chips or dice are becoming increasingly thinner. The separation distances between wires connected to the semiconductor dice are also becoming smaller. Moreover, more complicated structures such as stacked dice have been developed. In these cases, a high epoxy injection speed from conventional transfer molding will damage the dice or wires. Furthermore, complicated die and wire structures make it difficult for encapsulation material to perfectly fill a molding cavity.
To avoid the said wastage and to overcome the problems associated with molding delicate or complicated die and wire structures, one approach is to dispense liquid encapsulation material directly on top of the electronic devices to be molded, and then compressing the molding material to encapsulate the electronic devices and to form the desired shape of the electronic package.
An example of such a compression molding approach is disclosed in U.S. Pat. No. 6,743,389 entitled “Resin Molding Machine and Method of Resin Molding”. The resin molding machine therein comprises a lower die on which a work piece to be molded is set and an upper die clamping the work piece with the lower die. A clamper is provided to the upper die to enclose a resin molding space of the upper die, the clamper being capable of vertically moving in the upper die and always biased downward, wherein a lower end of the clamper is downwardly projected from a resin molding face of the upper die when the lower die and upper die are opened. A resin molding surface of the clamper compresses the molding compound during molding.
A problem faced with this approach is that the compression force is provided by the clamper's biasing spring force. Such spring force is of a limited range. As the package size increases, the molding cavity has to be made bigger and a greater compression force is therefore required to provide a larger compacting force by the clamper during molding of a greater volume of encapsulation material. In such cases, the spring force may be inadequate to reliably compress the molding compound to form a molded package. As a result, the molded product is defective. It would be desirable to develop a system for compression molding of electronic devices of larger package sizes that is more effective than solely using a spring's compression force to provide the compacting force.